A typical apparatus in these voltage monitoring apparatuses connects any unit battery in the battery pack to the capacitor, and charges the voltage across the selected unit battery into the capacitor. Thereafter, the apparatus connects the capacitor to a differential amplifier as an example of voltage converters, to cause the differential amplifier to amplify the difference in potential between both first and second ends of the capacitor. The differential amplifier outputs the amplified potential difference as an analog voltage to an analog-to-digital (A/D) converter, which is an example of voltage detectors. The A/D converter converts the analog voltage into a digital voltage as a measurement value of the selected unit battery in the battery pack.
In particular, the apparatus is configured to                (1) Connect the positive terminal of a first target unit battery in the battery pack to the first end of the capacitor while connecting the negative terminal of the target unit battery to the second end of the capacitor        (2) Connect the positive terminal of a second target unit battery adjacent to the first target unit battery in the battery pack to the second end of the capacitor while connecting the negative terminal of the unit battery to the first end of the capacitor.        
That is, the polarity of the voltage across the first target unit battery applied to the capacitor is opposite to that of the voltage across the second target unit battery, which is adjacent to the first target unit battery, applied to the capacitor. The BACKGROUND describes the polarity of the voltage across the first target unit battery applied to the capacitor as a positive polarity, and the polarity of the voltage across the second target unit battery applied to the capacitor as a negative polarity. For example, for the battery pack comprised of first to n-th series-connected unit batteries, the apparatus can be configured to                (1) Connect each of odd-numbered unit batteries to the capacitor in the positive polarity        (2) Connect each of even-numbered unit batteries to the capacitor in the negative polarity.        
The apparatus therefore requires the configuration that enables the A/D converter to convert each of the voltage across the capacitor in the positive polarity and the voltage across the capacitor in the negative polarity into a digital voltage.
Japanese Patent Application Publication No. 2005-292137 discloses such an apparatus in order to address such a requirement.
The published apparatus is designed in consideration of the voltage range, such as the voltage range from 0 to 5 V, which the A/D converter can handle, i.e. manipulate. Specifically, the published apparatus provides, to the differential amplifier, a reference voltage intermediate between the lower and upper limits of the voltage range, i.e. 2.5 V between 0 V and 5 V.
This configuration of the published apparatus converts a voltage across the capacitor having the positive polarity into a digital voltage within the first voltage range from the reference voltage to the upper limit, i.e. the first voltage range from 2.5 to 5 V. This configuration also converts a voltage across the capacitor having the negative polarity into a digital voltage within the second voltage range from the lower limit to the reference voltage, i.e. the second voltage range from 0 to 2.5 V.
This configuration therefore enables the A/D converter to convert each of a voltage across the capacitor in the positive polarity and a voltage across the capacitor in the negative polarity into a digital voltage.